Enable clang for modules failed with TLS.

New 3.8 clang/llvm can compile TLS code now.
* For x86_64, still disable clang due to f128 bug.
* For b/25643775, arm and arm64, disable clang in unit tests.
* Fix thread_local_test.cpp to compile with clang and
  limit gcc workaround only to arm and aarch64.

BUG: 25643775

Change-Id: Iecd006bf1fc417dbcce2c63343a59c4bf1fa77ea
diff --git a/tests/thread_local_test.cpp b/tests/thread_local_test.cpp
index aeba2ba..1422ed2 100644
--- a/tests/thread_local_test.cpp
+++ b/tests/thread_local_test.cpp
@@ -18,7 +18,8 @@
 #include <stdint.h>
 #include <string.h>
 
-#ifdef __GNUC__
+#if defined(__GNUC__) && !defined(__clang__) && \
+    (defined(__arm__) || defined(__aarch64__))
 // Gcc has a bug with -O -fdata-section for the arm target: http://b/22772147.
 // Until that bug is fixed, disable optimization since
 // it is not essential for this test.